CARBIDE-MATRIX INTERFACE MECHANISM OF STRESS-CORROSION CRACKING BEHAVIOR OF HIGH-STRENGTH CRMO STEELS

The effects of tempering temperature and carbon content on the stress corrosion cracking (SCC) behavior of high-strength CrMo steels in 3.5 pct NaCl aqueous solution have been studied by means of Auger electron spectroscopy (AES) and scanning and transmission electron microscopy (SEM and TEM). Experimental results show that the specimens with higher carbon content and tempered at lower temperatures have a higher tendency for intergranular fracture and lower threshold stress intensity K(ISCC). The SCC behavior is significantly affected by the distribution of carbide particles, especially carbide coverage on prior austenitic grain boundaries, through a carbide-matrix interface mechanism as the interface is the preferential site for the nucleation and propagation of microcracks because of its strong ability to trap hydrogen atoms. In low-temperature tempered states, there is the serious segregation of carbon in the form of carbide particles at prior austenitic grain boundaries, causing low-stress intergranular fracture. After tempering at high temperatures (greater-than-or-equal-to 400-degrees-C), both the coalescence of the carbide particles at the grain boundaries and the increase of carbide precipitation within grains cause the decrease of the tendency for intergranular fracture and the rise of K(ISCC). The higher the carbon content in steels, the more the carbide particles at the grain boundaries and, subsequently, the higher the tendency for low-stress intergranular fracture. The carbide effect on K(ISCC) makes an important contribution to the phenomenon that K(ISCC) decreases with the rise of yield strength of the steels.